Spray Polyurethane Foam Insulation Parameters

What is Spray Polyurethane Foam?

Today, thermal insulation is the factor with the greatest impact on energy savings. At this point, rigid polyurethane foam with a closed-cell structure is the material with the lowest heat transfer coefficient known in the world (0.018-0.022 W/mK). This type of polyurethane foam can be easily applied in spray form to surfaces requiring thermal insulation. When polyurethane is sprayed onto a surface, it adheres, expands, and creates a foam layer with a density of 20-40 kg/m3, providing effective thermal insulation.

How is Spray Polyurethane Foam Applied?

A spray machine is needed to apply this type of polyurethane foam. This machine draws polyol and isocyanate components from drums using pumps, heats them to 35-45°C, and pumps them under high pressure through polyol and isocyanate hoses. The hose is also heated to the same temperature to prevent the components from cooling. The polyol and isocyanate component hoses meet in the mixing chamber of the spray gun after a length of 15-30 m. These raw materials arriving under high pressure at the gun mix with each other the moment the trigger is pulled and are sprayed onto the surface with the help of pressurized air coming to the gun. When the polyol and isocyanate components mix, they react and begin to expand as soon as they hit the surface, creating the polyurethane foam structure. The polyurethane foam expanding within seconds creates an effective thermal insulation layer. 

Figure 1. Insulation with Spray Polyurethane Foam

Thermal Insulation of Spray Polyurethane Foam

Spray polyurethane foams expand with both a chemical blowing agent (water) and a physical blowing agent (low-boiling-point hydrocarbons). Since these foams are largely closed-cell, gases from these blowing agents (carbon dioxide and hydrocarbon gases) become trapped within the foam's cellular structure. At this point, heat transfer—the inverse of the foam's thermal insulation—is affected by three parameters below:
• Heat transfer through the polyurethane solid material
• Heat transfer through gases trapped in the cells
• The foam's density and cell size.

The heat transfer coefficients (ʎ) at room temperature of certain materials used in polyurethane foam structure are given in the table below. 

Table 1. Heat Transfer Coefficients of Some Materials in Foam Structure

Heat transfer through the polyurethane solid material is constant; however, at typical insulation densities (20-50 kg/m3), the polyurethane solid material comprises only 5% of the foam structure's volume. Therefore, the foam's heat transfer depends largely on the heat transfer of the blowing agent used. The choice of blowing agent is made considering its heat transfer coefficient, flammability, environmental regulations, and material cost. The foam's heat transfer increases with the foam's density (insulation decreases) because there is less space for the gases inside and radiation effect increases. The targeted heat transfer of polyurethane foam should be 0.020 W/m.K or less. 


Figure 2. Cell Structure of Rigid Polyurethane Foam

To achieve minimum heat transfer, the foam must have low density but its cells must be closed enough to prevent the blowing agent from escaping. Otherwise, the hydrocarbon gas is replaced by air, and as a result, the foam's heat transfer increases and insulation properties decrease. On the other hand, if the cells are completely closed (>95%) and the density is low (<25 kg/m3), rigid foam tends to shrink. To overcome the shrinkage problem at low densities, cells must be open at a high rate, which, as mentioned above, again increases heat transfer. Additionally, the foam's strength must be considered, since low-density foams are mechanically weak. In conclusion, insulation foam design is a matter of balance and optimization. The heat transfer coefficient and compression strength of a foam blown with cyclopentane for different densities are given below. 

Table 2. Heat Transfer Coefficient and Compression Strength Values of Rigid Foam

As seen from the table, if the foam is for suspended ceiling insulation (no foot traffic), the correct density for the foam is 20-25 kg/m3, and if the foam is for terrace insulation, the correct density is 30-40 kg/m3. Spray polyurethane foams have a density increase based on their free densities during application. Application density reaches a density 1.3-1.4 times the free density. That is, for 20-25 kg/m3 application density, the foam's free density should be 15-18 kg/m3. 

Figure 3. Spray Polyurethane Foam Application

Poleks Kimya produces foam systems in spray foams with free densities ranging from 12 kg/m3 to 40 kg/m3 for all types of insulation applications and assists applicators in selecting the correct density spray foam.